Polytetrafluoroethylene (hereinafter abbreviated as PTFE) is a substance having an extremely high degree of rubber elasticity such that even if it is heated to 380° C. above its melting point, it displays a melt viscosity of 1010 to 1012 Pa·S. Hence, unlike conventional resins, PTFE is not amenable to common shaping processes (e.g. melt forming) and is typically, for example, shaped by compressing a PTFE powder and heating the compact.
Currently, PTFE bulk powders are produced by polymerizing a tetrafluoroethylene monomer (hereinafter abbreviated as TFE) through chemical catalysis. The molecular chains in the resulting PTFE are straight-chained or linear, substantially free of branches, and have no crosslinked structure. The PTFE powder produced by suspension polymerization is called a molding powder whereas the PTFE powder produced by emulsion polymerization is called a fine powder.
Having a very high melt viscosity even at temperatures beyond its melting point, PTFE cannot be processed by shaping methods that are feasible with conventional thermoplastic resins. Hence, special processing methods are employed to make shaped articles of PTFE. In the case of the molding powder, it is commonly compressed into a shape, which is sintered and compacted, and then subjected to diverse finishing processes to make blocks, films, sheets, round bars, thick-walled pipes, and various cut articles.
In the case of the fine powder, it is commonly blended with an oil to form a homogeneous paste, which is shaped by extrusion and then dried and sintered into final products such as tubes, green tape, and filters.
Elongated round bars and tubes are sometimes produced by melt forming through ram extrusion molding (an application of compressive forming, in which filling with a powder, compressing, sintering, and cooling are continuously performed). However, ram extrusion molding differs from the ordinary melt extrusion molding in that the particulate resin is fed intermittently. Intermittent feeding of the resin necessitates repeated feeding and compressing of the resin, and an interface forms between stacked layers of the resin feed. In addition, the resin powder that is to be used in ram extrusion molding which needs to be automatically fed into the hopper is required to have high fluidity, so it is typically a granulated powder or a pre-sintered powder. As a result, resin particles often find difficulty in coalescing firmly under heating and the shaped articles have problems with strength and elongation. The failure to provide adequate coalescing has caused another problem, i.e., the occurrence of cracks and voids in the shaped articles.
Diaphragms may be formed by hot coining (i.e., a feed powder is filled into a mold and, after compressing the powder under heating at a temperature equal to or above its melting point, the powder is cooled for shaping as it is kept compressed); in this and other cases of making a variety of coated articles as well as impregnated products such as a glass cloth, a dispersion medium is used.
As mentioned earlier, PTFE bulk powders are currently produced by polymerizing TFE through chemical catalysis. The molecular chains in the resulting PTFE are straight-chained and substantially free of branches to feature a high degree of crystallinity. The degree of crystallinity of commercial PTFE bulk powders as produced by emulsion polymerization or suspension polymerization depends on their molecular weight but immediately after polymerization, they have more than 90% crystallinity irrespective of which method of polymerization was used; according to measurement with a differential scanning calorimeter, the commercial PTFE bulk powders have temperatures of crystal fusion at 340° C. and above, with an enthalpy of crystal fusion in excess of 55 J/g. Once these resins are sintered above their temperatures of crystal fusion, their melting points will shift to about 327° C. and their enthalpies of crystal fusion drop to 30 J/g and less. By sintering, the degree of crystallinity which was previously in excess of 90% drops to several tens of percent.
Some low-molecular weight PTFE powders can be obtained by exposing high-molecular weight PTFE bulk powders to a radiation in the air (in the presence of oxygen) so that the molecular chains are cut by radiolysis to lower the molecular weights of the bulk powders. In this case, too, the molecular chain in the PTFE has an unbranched, straight-chained structure.
The present inventors previously found that when a PTFE resin was exposed to 1 kGy or more of an ionizing radiation (hereinafter referred to simply as a radiation) at a temperature not lower than the melting point of the crystal of PTFE resin and in the absence of oxygen, crosslinking occurred to cause a great change in the characteristics of the PTFE resin (Patent document 1). Upon further scientific verification, the present inventors demonstrated that a Y-shaped higher-order structure was formed by crosslinking (Patent document 2). These inventions generally involve crosslinking a polymerized PTFE resin as a starting material, so that it is provided with a crosslinked structure to have wear resistance and creep resistance.
The present inventors also discovered that when a PTFE resin with a temperature, lower than its melting point was exposed to an electron beam (200 kGy to 10 MGy) at high dose rate (capable of imparting a large quantity of energy per unit time), the PTFE resin could be crosslinked even under such a condition that the initial resin temperature had not reached its melting point (Patent document 3).
The present inventors further discovered that when the radicals generated in PTFE upon exposure to a radiation (30-60 kGy) were subjected to graft polymerization with TFE, a Y-shaped higher-order structure was formed (Patent document 4).
Further in addition, the present inventors discovered that when a PTFE resin heated to a temperature lower than its melting point was exposed to a synchrotron radiation or high-intensity X-rays (1 kGy to 10 MGy) at high dose rate (capable of imparting a large quantity of energy per unit time), the PTFE resin could be crosslinked even under such a condition that the initial resin temperature had not reached its melting point (Patent document 5).
The present inventors also invented a method of producing an ultrafine PTFE powder with a particle size of no more than 1 μm by exposing TFE to a radiation (5-1000 kGy) in the solvent acetone (Patent document 6). The invention enables control over molecular weight under varied conditions so that ultrafine PTFE powders having a crosslinked structure can be produced over a wide range of molecular weights ranging from low to high values.
The present inventors further invented a method of producing PTFE with a crosslinked structure by exposing TFE to a radiation (10-1000 kGy) under low temperature (−196° C.). In this invention, too, scientific verification was made to demonstrate that a Y-shaped higher-order structure was formed in the resulting crosslinked PTFE (Patent document 7).
Ohshima et al. discovered that when a PTFE resin heated to a temperature lower than its melting point was exposed to an ion beam of high LET (linear energy transfer, or an energy imparted to a medium by radiation passing through it), the PTFE resin could be crosslinked even under such a condition that the initial resin temperature had not reached its melting point (Non-patent document 1).
The PTFE crosslinking related techniques described above are each directed to constructing a three-dimensional intermolecular structure by providing a solid crosslinked structure in the PTFE.
Techniques that have been developed for exposing PTFE to a radiation are such technology that high-molecular weight PTFE bulk powders are exposed to a radiation in the air (in the presence of oxygen) so as produce low-molecular weight PTFE powders. In this technology, the molecular chains are cut by radiolysis to lower the molecular weights of the bulk powders. As a matter of course, the produced low-molecular weight PTFE powder assumes an uncrosslinked, but straight-chained, molecular structure (JP 2008-69280 A, Patent document 9, Patent document 10, and Patent document 11).
Similarly, Patent document 13 describes a technique in which a once sintered shape is exposed to an ionizing radiation so that it can be stretched by an increased ratio; Patent document 13 describes a technique in which PTFE having higher deformability than the conventional PTFE shape is produced by irradiation; and Patent document 14 describes a technique in which a PTFE resin is exposed to an ionizing radiation so that a thin-walled PTFE tube can be prepared at high draw ratio; however, none of these techniques is directed to imparting a branched structure to the molecular chains. The PTFE obtained by these techniques has no more than a straight-chained molecular structure and its molecular weight has been reduced to such a level that the materials strength of PTFE will not be lost; those techniques are conceptually dissimilar to the present invention.
Patent document 15 provides a description of PTFE with a branched structure. However, the branched structure described in this publication is “macroscopic” in that it refers to the branching of fibers. In contrast, the branched structure as referred to in the present invention is at the molecular level which has a totally different meaning from the macroscopic structure.
PTFE can be shaped into fibers by various methods including the matrix spinning (or emulsion spinning) method, the splitting method, and the paste extruding method.
In the matrix spinning method, a liquid mixture of a PTFE dispersion and a matrix (which may be viscose) is extruded into a coagulating bath to form fibers and, thereafter, the greater part of the matrix is evaporated by firing and the PTFE is melted to coalesce, thereby producing PTFE fibers. Known techniques that may be classified as the matrix spinning method are described in Patent document 16, Patent document 17, and Patent document 18,
In the splitting method, a PTFE powder is compressed in a cylinder and the compact is sintered and subsequently split into filaments, which are then stretched. Known techniques that may be classified as the splitting method are described in Patent document 19, Patent document 20, Patent document 21, and Patent document 22.
In the paste extruding method, a PTFE powder is mixed with a petroleum-based oil or the like and the mixture is kneaded to form a paste which is extruded into a bar which, in turn, is trimmed, freed of the oil, and stretched or drawn. A known technique that may be classified as the paste extruding method is described in Patent document 23.
Other techniques that are classified as the paste extruding method are described in Patent document 24 and Patent document 25; a PTFE resin powder is blended with a petroleum-based oil or the like as an extrusion molding aid to make a compound, which is preliminarily shaped and the resulting preform is extruded as paste fibers, which are then dried, sintered, and drawn into PTFE resin fibers, provided that prior to the drawing step, the fibers are irradiated to be crosslinked. The method of making and processing fibers in these techniques consists of making a paste of the PTFE resin powder, shaping it into fibers, and drawing the fibers. This conventional method is yet to be sophisticated to enable the melt forming of PTFE.
Patent document 26 discloses a PTFE lamination technology but the PTFE films to be laminated have already been drawn and they are not sintered in the process of lamination.